Speaker
Description
The layered double hydroxides (LDH) represent a broad class of inorganic lamellar compounds with a high versatility in composition and large range of technological applications (1). We have studied the NiCuAl LDH where the charge of the trivalent ions ($Al^{3+}$) is compensated by carbonate anions in the inter-layer space (2). The coprecipitation method has been employed for the LDH preparation. It has investigated the effects of different synthesis parameters such as aging time, washing methods, and synthesis temperature, monitoring the effect on the long-range order of the final product. The formation of stacked layers in the synthesis can be clearly determined in LDH with TEM images. Moreover, we have carried out a study of the activation procedures, necessary for a catalytic application, involving a decomposition by heating procedure followed by reduction step in order to obtain a Cu/Ni nanoparticle finely and uniformly dispersed over alumina substrate. The particle size increase from ca. 3 nm, 3.5 nm, and 10 nm with increasing reduction temperature from 300 °C, 450 °C, 900 °C, respectively. The STEM-EDX mapping results illustrate the homogenous distribution of Ni and Cu nanosized particles after reduction procedures, suggesting an alloy formation. It has been found Ni reduction temperature is lowered by increasing content of Cu in LDH, indicating the addition of Cu could significantly promote Ni reducibility probably due to the hydrogen spillover effect. Moreover, the lower decomposition temperature (e.g., 290 °C) could also effectively promote the Ni reducibility. Furthermore, a deep XRD analysis illustrates that the observed lattice parameters match with the expected values obtained between Ni and Cu reference values (i.e. assuming Vegard type behaviour), further corroborating the Ni-Al alloy formation after reduction. The interaction between Cu and Ni is also visible in the near edge X-ray absorption fine structure (NEXAFS) spectra, where significant changes are observed in Cu L-edge and smaller change in Ni L-edge, pointing to alloy formation. The NEXAFS results show that the Cu L-edge is affected, both white line position and line shape, by presence of nickel. A smaller effect can also be detected on the Ni L-edge.
The activated materials different metal content; $Ni_{0.7}Al_{0.3}$, $Ni_{0.56}Cu_{0.14}Al_{0.3}$, $Ni_{0.35}Cu_{0.35}Al_{0.3}$; were tested for the reverse water gas shift (rWGS) reaction. Our catalytic results show that the material rich in Ni-content possesses the lowest for CO yield, but good stability described by an overgrowth around the particles that prevents the catalyst to sinter during catalysis. Both combinations of Ni-Cu catalysts, $Ni_{0.56}Cu_{0.14}Al_{0.3}$ and $Ni_{0.35}Cu_{0.35}Al_{0.3}$, present higher yield, but just for the material with Ni/Cu ratio of 4 a remarkable better stability was observed. The STEM-EDX results corroborated that the low stability observed for $Ni_{0.35}Cu_{0.35}Al_{0.3}$ sample is due to a remarkable Cu segregation.
References
- Bukhtiyarova, M. V. A review on effect of synthesis conditions on the formation of layered double hydroxides. J. Solid State Chem. 269, 494–506 (2019).
- De Roy, A. Lamellar Double Hydroxides. Mol. Cryst. Liq. Cryst. Sci. Technol. Sect. A. Mol. Cryst. Liq. Cryst. 311, 173–193 (1998).
